The disclosure relates to gas turbine engines. More particularly, the disclosure relates to rotors.
The disclosure relates to gas turbine engines. More particularly, the disclosure relates to rotors.
Turbine engine rotors include disk or ring components (collectively “disks”) that either: (1) have blade retention features that hold separately-formed blades (e.g., secured via convoluted “fir tree” blade roots and complementary disk slots) or (2) are formed integrally with the blade airfoils (so-called integrally bladed rotors (IBRs) or BLISKs or BLINGs).
In axial compressors, the disks are often formed of a high strength titanium alloy. The rotor may include a stack of such disks. They may be formed as integrally bladed rotors or as disks having a circumferential array of features such as fir tree or dovetail slots for receiving complementary blade roots.
In operation, as the rotor stack rotates, inertial forces stress the rotor stack. The rotation-induced tensile forces increase with radius. Exemplary engine speeds are 5,000-20,000 rpm for smaller engines and 10,000-30,000 rpm for larger engines. At high engine speeds, the inertial forces on outboard portions of a simple annular ring component could produce tensile forces in excess of the material strength of the component. It is for this reason that disk “bores” are ubiquitous in the art. The bore is a radially inboard protuberant annulus, coupled to the outer band (and thus such a “bore” is not to be confused with the central aperture of the disk which such bore typically bounds). The outer band carries or is integrally formed with the blade airfoils. A generally annular web extends radially outward from the bore to the band.
As noted above, the bore typically encircles a central aperture of the disk. A portion of a shaft may freely pass through the bore with clearance. The shaft may be of the same spool as the disk (e.g., in a center-tie rotor) or of a different spool.
By placing a large amount of material relatively inboard (and therefore subject to subcritical stress levels) some of the supercritical stress otherwise imposed on outboard portions of the disk may be transferred to the bore via the web. Thus, the disk with bore may be able to withstand a greater rotational speed than without.
Additionally, there may be hoop strength reinforcements for the outer band. Some of these may be sufficient to allow elimination of the bore. Numerous companies have explored the use of continuous fiber titanium metal matrix composites. See, e.g., U.S. Pat. No. 5,470,524. Ti MMC IBRs can save >25% in weight over conventional monolithic IBRs, largely by eliminating the bore. The drawbacks with continuous fiber MMC systems are related to the very high cost of the fiber, the labor intensive TMC fabrication process, and the non-isotropic properties of the MMC ring (which can lead to high levels of residual stress).
Separately, there has been work with powder metallurgy titanium with boron, carbon and silicon additions which results in a very fine TiB, TiC or TiSi2 precipitates that can increase strengths by 25-50% and modulus by 25%. Most of this work has been with Ti 6-4 (Ti-6 Al-4 V which is 6% Al and 4% V, by weight). Crucible Corp. has been making powder and FMW Composites Inc. has been doing the downstream processing. One particular example is shown in U.S. Pat. No. 7,531,021, the disclosure which is incorporated by reference in its entirety herein as is set forth at length. The precipitants form a discontinous reinforcement and, at least in the case of TiB referred to as whiskers, all with the notation TiBW. This is included in the use of the more simple term TiB below.
Recently, there has been work with higher strength titanium alloys. Strengths as high as 230 ksi (1.59 GPa) have been demonstrated with TiB reinforces Ti 5-5-5-3 (Ti-5 Al-5 V-5 Mo-3 Cr). This level of strength is similar to strengths demonstrated in conventional Ti MMCs. Other high strength and high temperature alloys (Ti 6-2-4-6, Ti 6-2-4-2, Alloy C+, and the like) could use a similar approach to produce very high strengths. Table I of FIG. 2 has physical properties of several conventional wrought alloys and TiB particulate-reinforced versions of the same alloys. Conventional long fiber Ti MMC properties are included for comparison. In the table, English units are the originals and metric units are calculated conversions.